1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus using laser light.
2. Description of the Related Art
In an image forming apparatus such as a laser printer, a digital copying machine, or a facsimile, it is general that light flux scans a scanning target surface while irradiating the scanning target surface so that a latent image is formed on the scanned surface.
The image forming apparatus includes a photosensitive element being photosensitive and serving as a scanned surface, a light source that emits laser light, a polygon mirror that deflects the laser light emitted from the light source, a scanning optical system that guides the laser light deflected by the polygon mirror to the surface of the photosensitive element, and the like.
An optical spot on the surface of the photosensitive element moves in an axial direction with the rotation of the polygon mirror, so that scanning corresponding to one line is performed. When scanning of one line is finished, the photosensitive element rotates to start the scanning of the following line. The axial direction of the photosensitive element is referred to as a “main scanning direction”, and the rotation direction of the photosensitive element is referred to as a “sub scanning direction”. The position of the optical spot on the surface of the photosensitive element in the main scanning direction is called an “image height”.
The scanning optical system is configured to include optical elements such as a lens, a glass plate, and a mirror. In scanning optical systems, however, light use efficiency (reflectance or transmittance) varies depending on the incident angle of light. Further, the thickness of the lens varies depending on the incident position of light.
The laser light deflected by the polygon mirror is incident to the scanning optical system at an incident angle depending on a deflection angle of the polygon mirror; and the incident position changes depending on the image height. Thus, the intensity of laser light on the surface of the photosensitive element becomes stronger or weaker according to the image height.
The dynamics in the laser light intensity according to the image height is called a “shading characteristic” and is one of factors that cause a density variation of an output image, thereby degrading the image quality. In this regard, there has been suggested various methods of correcting the shading characteristic (for example, see Japanese Patent Application Laid-open No. 2007-135100 and Japanese Patent Application Laid-open No. 2009-262344).
Further, an image forming apparatus that controls a light exposure amount according to a variation in sensitivity of the photosensitive element is disclosed in Japanese Patent Application Laid-open No. 2008-065270.
Meanwhile, when the photosensitive element is eccentric or has a cross section that is not a true circle, a gap between the photosensitive element and the developing roller varies as the photosensitive element rotates. A variation in the gap leads to a variation in development, causing an unnecessary density variation in an image (also referred to as “output image”) output from the image forming apparatus.
In recent years, there are demands for the high image quality, and conventional methods are difficult to suppress the density variation of the output image, caused by eccentricity or a shape error of the photosensitive element, down to a requested level.